LU-6142 tests: Use correct type for MGS

LU-6142 tests: Use correct type for MGS

When checking for MDS backend do use 'mds1_FSTYPE'
instead of mgs_FSTYPE which is not defined

Test-Parameters: trivial
Fixes: 3070ca9b1 (LU-4684 tests: enable racer directory migration)
Signed-off-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Change-Id: I11ae6956a03be9a076b7a5da72e9efc672230bdb
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56695
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>